Q.28 Arrange the following enzymes involved in pathway of fatty acids in
sequence :
A. Aetyl-CoA-acetyltransferase
B. Enoyl CoA hydratase
C. Acyl-CoA synthetase
D. Acyl-CoA dehydrogenase
E. ß -hydroxyacyl CoA dehydrogenase
Choose the correct answer from the options given below :
- C, B, D, E, A
- B, C, D, A, E
- B, D, C, A, E
- C, D, B, E, A
Fatty acid breakdown via β-oxidation in mitochondria follows activation → dehydrogenation → hydration → oxidation → thiolysis sequence, generating acetyl-CoA for TCA cycle. For Q.28—”Arrange enzymes: A. Acetyl-CoA acetyltransferase, B. Enoyl CoA hydratase, C. Acyl-CoA synthetase, D. Acyl-CoA dehydrogenase, E. β-hydroxyacyl CoA dehydrogenase”—correct answer is option 4: C, D, B, E, A.
Option Analysis
Option 1: C, B, D, E, A
Activation (C) first is correct, but hydration (B) before dehydrogenation (D) skips double bond formation, impossible without prior FAD-dependent removal of β-hydrogens.
Incorrect; wrong spiral cycle order.Option 2: B, C, D, A, E
Hydration (B) initiates wrongly—long-chain fatty acids require activation (C) and initial dehydrogenation first. Acetyltransferase (A) misplaced early.
Wrong; violates activation prerequisite.Option 3: B, D, C, A, E
Hydratase (B) before activation (C) defies biochemistry—free fatty acids cannot enter mitochondria without acyl-CoA thioester formation.
Incorrect; sequence starts with C.Option 4: C, D, B, E, A
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C: Acyl-CoA synthetase activates fatty acid + CoA + ATP → acyl-CoA (matrix transport via carnitine shuttle).
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D: Acyl-CoA dehydrogenase (FAD) forms trans-Δ²-enoyl-CoA.
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B: Enoyl CoA hydratase adds H₂O → L-β-hydroxyacyl-CoA.
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E: β-hydroxyacyl CoA dehydrogenase (NAD⁺) → β-ketoacyl-CoA.
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A: Acetyl-CoA acetyltransferase (thiolase) cleaves acetyl-CoA + shortened acyl-CoA; cycle repeats.
Correct; standard mitochondrial β-oxidation spiral.
Beta-Oxidation Cycle Overview
Step Enzyme Reaction Cofactor 1 C: Acyl-CoA synthetase Fatty acid → acyl-CoA ATP 2 D: Acyl-CoA dehydrogenase Acyl-CoA → enoyl-CoA FAD → FADH₂ 3 B: Enoyl-CoA hydratase Enoyl-CoA → 3-hydroxyacyl-CoA H₂O 4 E: Hydroxyacyl-CoA dehydrogenase 3-Hydroxyacyl-CoA → 3-ketoacyl-CoA NAD⁺ → NADH 5 A: Acetyl-CoA acetyltransferase 3-Ketoacyl-CoA → acetyl-CoA + acyl-CoA (n-2) CoA Yields 1 FADH₂ + 1 NADH per cycle; complete palmitate oxidation: 7 cycles → 8 acetyl-CoA.
Exam Relevance
CSIR NET/GATE Biochemistry core: Memorize C-D-B-E-A for β-oxidation (not synthesis, which uses FAS complex). Note: Unsaturated fats/peroxisomes modify pathway. Carnitine palmitoyltransferase enables entry.
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